In the Long-Term Evolution (LTE) wireless system standardized by the 3rd-Generation Partnership Project, a multi-access technique known as Single Carrier-Frequency Division Multiple Access (SC-FDMA) has been selected for uplink (mobile terminal-to-base station) transmissions. SC-FDCMA combines the desirable characteristics of Orthogonal Frequency Division Multiplexing (OFDM), which is used on the downlink, with a low Peak-to-Average Power Ratio (PAPR). This allows for the design of more efficient power amplifiers in the mobile terminals.
In an SC-FDMA scheme, as in an OFDM scheme, the used bandwidth is divided into a multitude of orthogonal subcarriers. SC-FDMA can be viewed as a Discrete-Fourier-Transform-coded OFDM, where time-domain data symbols are transformed to the frequency domain by a Discrete Fourier Transform (DFT) before going through OFDM modulation. Thus, while in OFDM groups of time-domain bits are mapped directly to one of the several subcarriers in the transmitted signal, in SC-FDMA each group of time-domain bits is spread across (typically) a subset of several subcarriers before the OFDM modulation is performed. This approach produces a single-carrier signal at the output of the SC-FDMA transmitter, rather than the multi-carrier signal of an OFDM transmitter.
Uplink Reference Signals in LTE
The LTE uplink (UL) incorporates two types of reference signals (RSs) for use in coherent data demodulation and channel sounding. These references signals are known as demodulation reference signals (DMRS) and sounding reference signals (SRS). DMRS are intended to be used by the LTE base station (known as an evolved NodeB, or eNB) for channel estimation for coherent demodulation of the uplink physical channels, which include the Physical Uplink Shared Channel (PUSCH) and Physical Uplink Control Channel (PUCCH). Thus, the DMRS is transmitted across the same bandwidth as the corresponding channel. In the case of PUSCH, two symbols per subframe are used for DMRS transmission, where the other symbols are used for PUSCH data transmission. The number of DMRS symbols for PUCCH per subframe depends on the PUCCH format.
In small-cell scenarios, mobile terminals (known as “user equipment” or “UEs” in 3GPP terminology) are expected to experience a relatively small degree of channel selectivity, in both the frequency and time domains. This suggests the possibility of reducing the transmission of DMRS in the uplink, thus reducing overhead and improving spectral efficiency. One approach to overhead reduction is to drop one DMRS symbol per subframe, i.e., using only one DMRS symbol per subframe. This approach is perceived to be a way to achieve higher spectral efficiency for small cell scenarios with reasonable implementation complexity.
SRS are transmitted on the UL to allow the eNodeB to estimate the uplink channel state. An SRS is not necessarily transmitted together with any physical channel. If transmitted together with, for example PUSCH, the SRS may cover a different, typically larger, frequency span. Two types of SRS transmission are defined for LTE uplink: periodic and aperiodic SRS transmission.
Periodic SRS transmissions from a UE occur at regular time intervals. The SRS transmission resources/parameters are semi-statically configured via Radio Resource Control (RRC) signaling. In the specific subframes identified by this configuration, a UE transmits SRS in the last symbol, across the entire frequency band of interest with a single SRS transmission or across part of the band with hopping in the frequency domain.
In contrast, aperiodic SRS are usually “one-shot” transmissions, triggered by signaling sent on the Physical Downlink Control Channel (PDCCH) as part of the scheduling grant. In the same way as for periodic SRS transmission, aperiodic SRS are transmitted within the last symbol of a subframe across the entire frequency band, or across part of the band with frequency hopping. Furthermore, aperiodic SRS transmissions can be configured for a specific UE for a duration N, such that the UE transmits SRS in each of the N next UE-specific SRS subframes. This is referred as “multi-shot” SRS. SRS transmission in multiple subframes may be useful for supporting frequency hopping. Given that a trigger may result in more than one SRS transmission, transmission in consecutive subframes can be considered.
Frequency Offset in LTE
As discussed above, SC-FDMA is used for the LTE uplink. With this scheme, the used bandwidth is divided into a multitude of orthogonal subcarriers. However, the orthogonality of the subcarriers is sensitive to the effects of frequency offset, which is a mismatch between a receiver's reference frequency and the carrier. Frequency offset at the eNodeB can have several causes, including Doppler shift resulting from the mobile terminal's motion, as well as any inaccuracy of the carrier frequency in the UE.
Typically, the mobile terminal locks its oscillator frequency to the downlink signal, which may be affected by a Doppler shift from the mobile terminal's motion. In the uplink, the signal is again subject to a Doppler shift, resulting in twice the frequency offset, compared to the offset in the downlink direction. Thus, the maximum total frequency offset due to the Doppler shift is approximately
            f      offset        =                  2        ×                  f          d                    =              2        ⁢                              v            ×                          f              c                                c                      ,where fc is the carrier frequency.
According to the 3GPP specification 3GPP TS36.101, the standardized accuracy of the carrier frequency for a UE transmission is ±0.1 ppm, which yields an additional fcarrier_accuracy=0.1×10−6×fc of frequency offset. This must be considered in addition to the Doppler-induced frequency offset. Because these offsets are a fraction of the carrier frequency, at large carrier frequencies the absolute frequency offset can be quite large, relative to the subcarrier spacing.
The effect of the frequency offset is that the subcarriers in the received signal at the eNodeB are no longer orthogonal, which results in inter-subcarrier interference. Especially for small-cell scenarios, where the carrier frequency can be up to 3.5 GHz, it is important to accurately estimate the frequency offset at the network, so that compensation can be performed. Also, channel estimation performance may suffer unless a frequency offset is properly compensated, since the channel will change at a more rapid rate over time than without a frequency offset, thus making channel interpolation more difficult.
Accordingly, consistent and reliable techniques for estimating frequency offset in LIE networks are needed. Furthermore, these techniques must remain consistent and reliable even as the LTE specifications evolve to support new modes of operation.